Three-Phase Motor Power and Torque Calculator
Estimate three-phase motor input power, shaft horsepower, and torque from matching motor-terminal measurements and nameplate data.
Electrical input and shaft details
Enter line-to-line RMS voltage at the motor terminals.
Use current measured or specified for the same operating condition.
Enter a decimal from 0 to 1 from matching motor data or measurement.
Use the efficiency for the same voltage, load, and operating condition.
Use measured speed or the motor's rated speed for the entered operating point.
Estimated shaft output
About this calculator
Enter line voltage, line current, power factor, motor efficiency, and shaft speed for one operating condition. The calculator estimates electrical input power and mechanical output at the shaft.
Use motor-terminal data for a balanced three-phase motor. Do not mix readings from different load conditions or use VFD input/output measurements.
Method and assumptions
Apparent input power = √3 × line voltage × line current. Real input power = apparent power × power factor. Estimated shaft output = real input power × motor efficiency. Horsepower uses the mechanical horsepower conversion. Shaft torque = horsepower × 33,000 ÷ (2π × shaft speed in rpm).
The calculation assumes balanced sinusoidal three-phase operation and uses the entered values as a matched set. It does not account for voltage or current distortion, phase imbalance, drive losses, or changing motor efficiency with load.
Interpret the result
Use the result to compare an estimated operating point with equipment documentation or measurements. It is not a motor selection, circuit sizing, or protection decision. Verify the motor and driven equipment against their data sheets and the applicable installation requirements.
FAQs
Which voltage should I enter?
Enter line-to-line RMS voltage measured or specified at the motor terminals. Use values that correspond to the same operating condition as the current, power factor, and efficiency.
Can I use readings from a VFD?
No. This calculation assumes balanced sinusoidal motor-terminal voltage and current. VFD input and output readings include drive effects that this worksheet does not model.
How is shaft torque estimated?
The calculator converts estimated shaft horsepower and the entered shaft speed to torque. Use measured speed or a matching rated speed; torque changes with speed for the same shaft power.
Does this show whether the motor is correctly sized?
No. It estimates one operating point only. It does not evaluate starting torque, load curves, service factor, thermal limits, circuit conductors, overload protection, or the motor application.
Sources
- NIST Guide to the SI, Appendix B.9 — conversion factors for mechanical horsepower and pound-foot.
- Use the motor manufacturer’s data for power factor, efficiency, rated speed, and operating limits.